U.S. patent application Ser. No. 09/048,402 filed Mar. 25, 1998 in the names of K. B. Roberts et al. (corresponding to UK patent application No. 9802913.5 filed Feb. 11, 1998) entitled xe2x80x9cMultiplexed Transmission Of Optical Signalsxe2x80x9d relates to high capacity optical transmission systems in which optical signals are multiplexed using a waveguide array to provide a relatively large number of transmission channels and hence a very high transmission capacity of the order of 1 Tb/s (one terabit, or 1012 bits, per second). The entire disclosure of this United States patent application is hereby incorporated herein by reference.
In an array transmission system as described in the application referred to above, optical pulses from a laser source are split into a large number of channels by a waveguide array, and optical signals of the channels are modulated by respective modulators and are combined by another waveguide array to be communicated over an optical fiber path. Such a system can provide high spectral density, but requires a relatively large number of modulators.
It is known that optical fiber chromatic dispersion is a limiting factor for transmission distance in high speed optical communications systems. Another important criterion in an array transmission system as discussed above is the implementation of the system, particularly in relation to the costs and technical complexity and risks associated with the modulators.
It would be desirable to avoid the need for external modulators by providing direct modulation of a semiconductor laser. Amplitude modulation (AM) of the intensity of the optical signal produced by the laser enables direct detection at an optical receiver to recover the original binary signal. However, direct AM of a semiconductor laser results in the optical signal having a spectral occupancy, or frequency chirp, that is not acceptable for long distance transmission due to the chromatic dispersion of the fiber.
This difficulty can be addressed by performing direct frequency modulation (FM) of the semiconductor laser, and converting the resulting FM optical signal to an AM optical signal using a bandpass optical filter, for example as described in xe2x80x9c5 Gbit/s Optical FSK Modulation Of A 1530-nm DFB Laserxe2x80x9d by R. S. Vodhanel et al., European Conference on Optical Communication, 1988. As described there, a DFB (distributed feedback) laser is modulated with a pseudo-random NRZ (non-return to zero) binary signal and the resulting FSK (frequency shift keyed, i.e. FM) optical signal is conducted via an etalon and an optical fiber to a p-i-n photodiode detector, the etalon serving to perform FSK demodulation.
While this provides improved performance compared with direct AM, it remains inadequate for long distance transmission. In particular, the above article recognizes that the laser has a non-uniform low-frequency FM response which can distort the optical FSK signal and produce errors in the communicated signal. This non-uniformity is understood to be a result of thermal frequency shift of the laser, and particularly affects that part of the spectrum of the signal being transmitted that is below a frequency of about 20 MHz.
xe2x80x9cBipolar Optical FSK Transmission Experiments at 150 Mbit/s and 1 Gbit/sxe2x80x9d by R. S. Vodhanel et al., Journal of Lightwave Technology, Vol. 6, No. 10, October 1988, pages 1549-1553, mentions various modulation techniques, such as Manchester coding, proposed to eliminate unwanted thermal frequency modulation of semiconductor lasers, and proposes using a bipolar signal format for this purpose. The bipolar signal has a signal power or energy which decreases towards zero for low frequency signal components towards zero frequency, so that the undesired thermal frequency modulation of the laser at low frequencies is reduced. However, the optical receiver is much more complicated, in this case requiring a frequency discriminator for demodulation and using a Schmitt trigger to convert the demodulated signal from the bipolar format back to its original NRZ form. In addition, Manchester coding or this bipolar format increases the spectral occupancy of the resulting optical signal, which as discussed above results in reduced performance for long distance transmission due to chromatic dispersion.
It is known, for example from Yonenaga et al., xe2x80x9cDispersion-Tolerant Optical Transmission System Using Duobinary Transmitter and Binary Receiverxe2x80x9d, Journal of Lightwave Technology, Vol. 15, No. 8, August 1997, pages 1530-1537, and from Yonenaga et al. U.S. Pat. No. 5,543,952 issued Aug. 6, 1996 and entitled xe2x80x9cOptical Transmission Systemxe2x80x9d, to use duobinary code for a modulating signal supplied in push-pull manner to a dual-drive Mach-Zehnder (MZ) type optical intensity modulator in an optical communications system. The use of duobinary code in this manner reduces the signal bandwidth required for a given signal transmission rate, and permits direct detection to recover the original binary signal at an optical receiver. Such an arrangement again requires an external modulator and involves the costs and risks associated therewith especially in an array transmission system. For example, cross-talk of high voltage, high frequency signals among closely spaced electrical circuits presents a significant problem, and modulation using duobinary encoded signals as disclosed by Yonenaga et al. doubles the voltage swings of signals supplied to the modulators, thereby exacerbating this problem.
An alternative duobinary encoding technique is described in International patent application PCT/CA98/00275 by Northern Telecom Limited, published Oct. 8, 1998 under No. WO 98/44635 and entitled xe2x80x9cDuobinary Coding And Modulation Technique For Optical Communication Systemsxe2x80x9d.
The article by Yonenaga et al. referred to above also refers to a dispersion-supported transmission (DST) technique, as disclosed by B. Wedding et al., xe2x80x9c10-Gb/s optical transmission up to 253 km Via Standard Single-Mode Fiber Using the Method of Dispersion-Supported Transmissionxe2x80x9d, Journal of Lightwave Technology, Vol. 12, No. 10, October 1994, pages 1720-1727. The DST technique uses direct modulation of a laser diode with a NRZ binary signal to produce an FSK optical signal, and FM-AM conversion in the dispersive optical fiber with direct detection of the AM component at an optical receiver. Consequently, the DST technique requires the frequency deviation of the FSK optical signal to be adjusted, depending upon the chromatic dispersion of the fiber, to match the group delay between the FSK components to the bit duration. In addition, recovery of the NRZ binary signal from the detected AM component of the converted optical signal requires additional processing, for example by an integrator and a decision circuit.
This invention seeks to facilitate optical signal transmission of high speed signals over long distances, with relatively low technical complexity and cost, in a manner that can be suitable or advantageous for use for array transmission.
One aspect of this invention provides a method of producing an amplitude modulated optical signal representing a binary signal, comprising the steps of: encoding the binary signal to produce a three-level encoded signal, the encoded signal having two outer levels representing a first state of the binary signal and having an intermediate level representing a second state of the binary signal; producing an optical signal frequency modulated in accordance with the three-level encoded signal; and optically converting the frequency modulated optical signal in dependence upon its frequency to produce an amplitude modulated optical signal having first and second amplitudes representing the first and second states of the binary signal.
Preferably the step of producing the frequency modulated optical signal comprises direct modulation of a semiconductor laser by the encoded signal.
The step of encoding preferably encodes the binary signal in accordance with a polynomial having factors (1xe2x88x92D) and (1+D) where D is a delay operator for the binary signal, and conveniently comprises modified duobinary encoding and precoding of the binary signal. The encoded signal consequently has reduced (halved) bandwidth compared with the binary signal, substantially zero d.c. component, and relatively little low frequency energy. This enables the generation of an optical signal with reduced spectral occupancy and substantially no carrier frequency component, and the problem of low-frequency non-uniformity of the direct-modulated laser is substantially reduced.
Preferably the step of optically converting the frequency modulated optical signal comprises conducting the optical signal via an interference filter having two paths producing constructive and destructive interference for different optical signal frequencies representing the two states of the binary signal. One and zero bits of the binary signal can be represented by respective frequencies of the frequency modulated optical signal for which the interference filter provides constructive and destructive interference respectively, to facilitate direct recovery of the binary signal from the optical signal as received at an optical receiver. The method desirably includes the step of controlling a central frequency of the frequency modulated optical signal for maximum destructive interference in the interference filter for zero bits of the binary signal.
Conveniently the two paths of the interference filter provide a differential time delay of the optical signal which is of the same order as a bit period of the binary signal.
Another aspect of the invention provides a method of transmitting a plurality of binary signals, comprising producing a plurality of optically multiplexed amplitude modulated optical signals, each representing a respective one of the plurality of binary signals, each by the above method, wherein the steps of encoding the binary signal and producing the frequency modulated optical signal are carried out individually for the respective binary signals to produce respective frequency modulated optical signals having different central frequencies, the method including a step of optically combining the frequency modulated optical signals, and wherein the step of optically converting the frequency modulated optical signals is carried out on the combined frequency modulated optical signals using a single interference filter.
Preferably the step of optically combining the frequency modulated optical signals comprises optically filtering the frequency modulated optical signals in accordance with respective frequency channels.
A further aspect of the invention provides a method of producing an amplitude modulated optical signal representing a binary signal, comprising the steps of: encoding the binary signal to produce a three-level encoded signal, the encoded signal having two outer levels representing a first state of the binary signal and having an intermediate level representing a second state of the binary signal; producing an optical signal with a polarization that is modulated in accordance with the three-level encoded signal; and filtering the optical signal in dependence upon its polarization to produce an amplitude modulated optical signal having first and second amplitudes representing the first and second states of the binary signal.
Thus polarization modulation and a polarization filter can be used instead of frequency modulation and an interference filter. In this case preferably one and zero bits of the binary signal are represented by orthogonal polarizations of the modulated optical signal for which the step of filtering provides relative transmission and attenuation respectively.
The invention also provides an optical signal transmitter comprising: an encoder for encoding a binary signal to produce a three-level encoded signal, the encoded signal having two outer levels representing a first state of the binary signal and having an intermediate level representing a second state of the binary signal; a semiconductor laser directly modulated by the encoded signal to produce a frequency modulated optical signal; and an optical interference filter to which the frequency modulated optical signal is supplied, the interference filter producing an amplitude modulated optical signal by constructive and destructive interference at different frequencies of the frequency modulated optical signal.
In addition, the invention provides an optical transmission system comprising: a plurality of N encoders each for encoding a respective binary signal to produce a respective three-level encoded signal having reduced bandwidth and d.c. components compared with the respective binary signal; N semiconductor lasers each directly modulated by a respective encoded signal to produce a respective one of N frequency modulated optical signals having different frequency ranges; an optical combiner arranged to combine the frequency modulated optical signals in their respective frequency ranges; and an optical interference filter to which the combined frequency modulated optical signals are supplied, the interference filter producing in each of said respective frequency ranges an amplitude modulation of the optical signal by constructive and destructive interference at different frequencies of the respective frequency modulated optical signal, the amplitude modulation providing two states representing the binary states of the respective binary signal.
Furthermore, the invention provides an optical signal transmitter comprising: an encoder arranged to encode a binary signal to produce a three-level encoded signal having reduced bandwidth and d.c. components compared with the binary signal; a modulator arranged to modulate polarization of an optical signal in accordance with the three-level encoded signal; and a polarization filter arranged to filter the optical signal in dependence upon its polarization to produce an amplitude modulated optical signal having first and second amplitudes representing the first and second states of the binary signal.
The invention also extends to all useful, novel, and inventive combinations and sub-combinations of the various features disclosed and/or claimed individually and/or collectively herein.